In recent years, a substrate-type optical waveguide having a phase modulation function has been widely used as a part of an optical modulator which is mounted on an optical transmitter. For example, a substrate-type optical waveguide which includes a silicon on insulator (SOI) substrate as a base includes (1) a base layer containing silicon (Si) as a main component, (2) a lower cladding layer which is laminated on the base layer and contains silica (SiO2) as a main component, and (3) a core which is provided on the lower cladding layer and contains silicon (Si) as a main component.
The core of the substrate-type optical waveguide having the phase modulation function includes an N-type semiconductor region, a P-type semiconductor region, and a depletion layer that exists between those regions. An effective refractive index of the core is determined in accordance with a size of the depletion layer, and the size of the depletion layer can be changed in accordance with a magnitude of a voltage which is applied between the N-type semiconductor region and the P-type semiconductor region. Therefore, it is possible to modulate a phase of light propagating through the core by applying a signal voltage between the N-type semiconductor region and the P-type semiconductor region.
For example, Patent Literatures 1 and 2 disclose substrate-type optical waveguides having such a phase modulation function. In each of the substrate-type optical waveguides disclosed in Patent Literatures 1 and 2 (“phase shifter” in Patent Literature 1 and “optical Semiconductor device” in Patent Literature 2), a rib-slab type care (“wave guide” in Patent Literature 1 and “semiconductor layer” in Patent Literature 2) is used. Here, the rib-slab type core is a core having an inverted T-shaped cross section (which is a cross section perpendicular to a traveling direction of light propagating through the core), that is, a core having a rib part which is relatively greater in height (i.e., greater in thickness) and slab parts which are located on both sides of the rib part and are relatively smaller in height (i.e., smaller in thickness).
In the rib-slab type core, light guided therein is concentrated on the rib part. From this, in a case where the N-type semiconductor region and the P-type semiconductor region are arranged such that the depletion layer exists in the rib part, it is possible to reduce a voltage (hereinafter, referred to as “operating voltage”) Vpi which is to be applied between the N-type semiconductor region and the P-type semiconductor region in order to control an effective refractive index of the core to a predetermined value, and it is thus possible to reduce power consumption.